CT scanners are well known in the art. Generally, such scanners comprise an X-ray tube, mounted on an annular gantry, so as to revolve about a subject being imaged. The subject lies on a bed, which is translated through the gantry. The axis of translation of the bed (conventionally the Z-axis) is generally parallel to the long axis of the subject's body, which is typically perpendicular to the plane of revolution of the tube.
An array of X-ray detectors on the opposite side of the subject from the X-ray tube receive radiation transmitted through the subject. The detectors generate signals proportional to the attenuated X-ray flux incident thereon, corresponding to a series of circumferentially-disposed angular “views” through the subject. These signals are pre-processed to produce attenuation data, which are used in reconstructing a three-dimensional image of the subject. In “third-generation” scanners, the array of detectors is mounted on the gantry so as to revolve along with the X-ray tube, whereas in “fourth-generation” scanners, the detectors are arrayed in a ring, which is generally stationary.
CT scanners generally operate in axial or helical scan modes. In axial modes, the gantry and the bed move in alternation, i.e., the bed is held stationary at a given axial position while the gantry revolves about the subject. After a desired number of full or partial revolutions, the bed is advanced to the next, generally adjacent, axial position, and the gantry revolutions are repeated, thus continuing until all or a selected portion of the subject's body is scanned and corresponding image slices are reconstructed. In helical modes, the gantry revolves and the bed advances simultaneously, so that the X-ray tube describes a generally helical path relative to the body.
In a helical-mode scanner, in order to reconstruct a planar cross-sectional image slice of the subject at a desired axial position, based on the helical-scan views, effective attenuation values for each of a plurality of points around a circumference of such a planar slice are derived by interpolation between data received in the original helical-path views. For each of the plurality of points, the respective effective attenuation values correspond to the approximate attenuation along rays within the planar slice that pass through the point. For 360° reconstruction, as is known in the art, the plurality of points are distributed around the entire circumference of the slice, whereas for 180° reconstruction, also known in the art, the points are distributed on a half-circumference. (For convenience in the following discussion, we will refer to the total angular extent of all the views that are collectively used in the reconstruction of a complete planar slice as the “reconstruction angle,” typically 360° or 180°.) The interpolated data are filtered and back-projected to produce the cross-sectional image.
Cross-sectional images thus produced by CT scanners generally lag behind the acquisition of the attenuation data by several seconds at the least. This lag stems from several factors, including (1) the necessity of receiving data from views over the entire reconstruction angle (or more, in the case of helical scanners) before reconstructing the image; and (2) the time needed to complete the intensive computations involved in back-projecting an entire image slice. The lag is particularly disadvantageous when CT imaging is used to track the progress of a physiological process, such as the flow of a contrast material. Similarly, when the CT scanner is used to guide a surgical procedure, such as a biopsy, the surgeon receives visual feedback regarding his progress in the procedure with a delay of more than one scan period.
Multi-slice axial and helical-path scanners are known in the art. For example, U.S. Pat. No. 5,485,493, which is incorporated herein by reference, describes a multiple-detector-ring spiral scanner with relatively adjustable helical paths, in which two adjacent, parallel slices are acquired along two parallel paths simultaneously or sequentially. Data corresponding to planar slices are derived by interpolating between data acquired along the two helical paths.
U.S. Pat. No. 5,524,130, the disclosure of which is incorporated herein by reference describes a number of methods for utilizing a single detector ring scanner to provide successive axially spaced slices with reduced time between reconstruction of the slices. Some of these methods appear to utilize partial scan data from one scan to replace data from a second scan for reducing the reconstruction time.